I just returned from a trip STL-MCO-STL this past week on WN. Both of the aircraft that I flew on were 733s. The first aircraft's engine noises were very loud and whiny, and you could easily hear the engine starting and spooling up. On the return leg, the aircraft's engines were much quieter and sounded very windy. What would cause such different engine noises on the same type of aircraft and airline? Has Southwest been upgrading their 733 engines?
Thanks for all of your help!